In the context of an infinitely repeated capacity-constrained price game, we endogenize the composition of a cartel when .rms are heterogeneous in their capacities. When .rms are sufficiently patient, there exists a stable cartel involving the largest .rms. A .rm with sufficiently small capacity is not a member of any stable cartel. When a cartel is not all-inclusive, colluding firms set a price that serves as an umbrella with non-cartel members pricing below it and producing at capacity. Contrary to previous work, our results suggest that the most severe coordinated e¡èects may come from mergers involving moderate-sized firms, rather than the largest or smallest firms.